OFDM and OFDMA wireless communication systems under IEEE 802.16 use a network of base stations to communicate with wireless devices (i.e., mobile stations) registered for services in the systems based on the orthogonality of frequencies of multiple subcarriers and can be implemented to achieve a number of technical advantages for wideband wireless communications, such as resistance to multipath fading and interference. Each base station (BS) emits and receives radio frequency (RF) signals that convey data to and from the mobile stations (MS). Such an RF signal from a BS includes an overhead load, in addition to the data load (voice and other data), for various communication management functions. Each MS processes the information in the overhead load of each received signal prior to processing the data.
Under the current versions of the IEEE 802.16 standard for the OFDMA systems, every downlink subframe from a base station includes a preamble and a frame control header (FCH) following the preamble as part of the overhead load. The preamble includes information for searching a cell and a cell sector within a cell and for synchronizing a mobile station in both time and frequency with the received downlink signal. The FCH portion of the downlink subframe includes information on the downlink transmission format (e.g., the downlink media access protocol, or DL MAP) and control information for the downlink data reception (e.g., allocation of the subcarriers in the current downlink frame). Therefore, a receiver, such as a MS, first decodes the FCH to determine the position of the DL MAP, decodes the DL MAP of the corresponding position, and then extracts the data.
If the communication quality falls below a certain threshold, a MS may start scanning for another BS with which to execute a hand-over (HO). However, under the 802.16e standard, a MS should stop transmission and reception of data to scan neighboring base stations. Accordingly, to scan for another BS, a MS may request a serving BS to allocate time intervals during which the MS may scan neighboring BSs. A MS may scan neighboring BSs by sending a MOB_SCN-REQ message in which requested scan duration, interleaving interval, and scan information may be included.
A serving BS that has received a MOB_SCN-REQ may grant time intervals to the MS by sending a MOB_SCN-REP message which may include a scanning start frame and the values granted. Additionally, the BS may send unsolicited MOB_SCN-RSP messages to trigger the MS to begin neighbor BS scanning.
Neighbor BS scanning is an essential function for a MS to effectuate a proper HO. It is evident that more frequent scanning of BS will improve HO performance. However, under the 802.16e standard, a MS should stop transmission and reception of data to scan neighbor base stations, meaning the BS should not send data to the MS during a scanning interval and the BS is not responsible for receiving data from the MS during a scanning interval.